History of geodesy
, also named geodetics, is the scientific discipline that deals with the measurement and representation of the Earth. The history of geodesy began in pre-scientific antiquity and blossomed during the Age of Enlightenment.
Early ideas about the figure of the Earth held the Earth to be flat, and the heavens a physical dome spanning over it. Two early arguments for a spherical Earth were that lunar eclipses were seen as circular shadows which could only be caused by a spherical Earth, and that Polaris is seen lower in the sky as one travels South.
Hellenic world
The early Greeks, in their speculation and theorizing, ranged from the flat disc advocated by Homer to the spherical body postulated by Pythagoras. Pythagoras's idea was supported later by Aristotle. Pythagoras was a mathematician and to him the most perfect figure was a sphere. He reasoned that the gods would create a perfect figure and therefore the Earth was created to be spherical in shape. Anaximenes, an early Greek philosopher, believed strongly that the Earth was rectangular in shape.Since the spherical shape was the most widely supported during the Greek Era, efforts to determine its size followed. Aristotle reported that mathematicians had calculated the circumference of the Earth to be 400,000 stadia while Archimedes stated an upper bound of 3,000,000 stadia using the Hellenic stadion which scholars generally take to be 185 meters or of a geographical mile.
Hellenistic world
In Egypt, a Greek scholar and philosopher, Eratosthenes measured Earth's circumference with great precision. He estimated that the meridian has a length of 252,000 stadia, with an error on the real value between -2.4% and +0.8%. Eratosthenes described his technique in a book entitled On the measure of the Earth, which has not been preserved.is on the Tropic of Cancer and on the same meridian as Alexandria
Eratosthenes' method to calculate the Earth's circumference has been lost; what has been preserved is the simplified version described by Cleomedes to popularise the discovery. Cleomedes invites his reader to consider two Egyptian cities, Alexandria and Syene, modern Assuan:
- Cleomedes assumes that the distance between Syene and Alexandria was 5,000 stadia ;
- he assumes the simplified hypothesis that Syene was precisely on the Tropic of Cancer, saying that at local noon on the summer solstice the Sun was directly overhead;
- he assumes the simplified hypothesis that Syene and Alexandria are on the same meridian.
Eratosthenes' method was actually more complicated, as stated by the same Cleomedes, whose purpose was to present a simplified version of the one described in Eratosthenes' book. The method was based on several surveying trips conducted by professional bematists, whose job was to precisely measure the extent of the territory of Egypt for agricultural and taxation-related purposes. Furthermore, the fact that Eratosthenes' measure corresponds precisely to 252,000 stadia might be intentional, since it is a number that can be divided by all natural numbers from 1 to 10: some historians believe that Eratosthenes changed from the 250,000 value written by Cleomedes to this new value to simplify calculations; other historians of science, on the other side, believe that Eratosthenes introduced a new length unit based on the length of the meridian, as stated by Pliny, who writes about the stadion “according to Eratosthenes' ratio”.
A parallel later ancient measurement of the size of the Earth was made by another Greek scholar, Posidonius. He noted that the star Canopus was hidden from view in most parts of Greece but that it just grazed the horizon at Rhodes. Posidonius is supposed to have measured the angular elevation of Canopus at Alexandria and determined that the angle was 1/48 of circle. He used a distance from Alexandria to Rhodes, 5000 stadia, and so he computed the Earth's circumference in stadia as 48 times 5000 = 240,000. Some scholars see these results as luckily semi-accurate due to cancellation of errors. But since the Canopus observations are both mistaken by over a degree, the "experiment" may be not much more than a recycling of Eratosthenes's numbers, while altering 1/50 to the correct 1/48 of a circle. Later, either he or a follower appears to have altered the base distance to agree with Eratosthenes's Alexandria-to-Rhodes figure of 3750 stadia since Posidonius' final circumference was 180,000 stadia, which equals 48×3750 stadia. The 180,000 stadia circumference of Posidonius is suspiciously close to that which results from another method of measuring the earth, by timing ocean sunsets from different heights, a method which is inaccurate due to horizontal atmospheric refraction.
The above mentioned larger and smaller sizes of the Earth were those used by Claudius Ptolemy at different times, 252,000 stadia in his Almagest and 180,000 stadia in his later Geographia. His mid-career conversion resulted in the latter work's systematic exaggeration of degree longitudes in the Mediterranean by a factor close to the ratio of the two seriously differing sizes discussed here, which indicates that the conventional size of the earth was what changed, not the stadion.
Ancient India
The Indian mathematician Aryabhata was a pioneer of mathematical astronomy. He describes the earth as being spherical and that it rotates on its axis, among other things in his work Āryabhaṭīya. Aryabhatiya is divided into four sections. Gitika, Ganitha, Kalakriya and Gola. The discovery that the earth rotates on its own axis from west to east is described in Aryabhatiya. For example, he explained the apparent motion of heavenly bodies is only an illusion, with the following simile;Aryabhatiya also estimates the circumference of Earth. He gave the circumference of the earth as 4967 yojanas and its diameter as 1581+1/24 yojanas. The length of a yojana varies considerably between sources; assuming a yojana to be 8 km this gives as circumference nearly 39,736 km
Roman Empire
In late antiquity, such widely read encyclopedists as Macrobius and Martianus Capella discussed the circumference of the sphere of the Earth, its central position in the universe, the difference of the seasons in northern and southern hemispheres, and many other geographical details. In his commentary on Cicero's Dream of Scipio, Macrobius described the Earth as a globe of insignificant size in comparison to the remainder of the cosmos.Islamic world
The Muslim scholars, who held to the spherical Earth theory, used it to calculate the distance and direction from any given point on the earth to Mecca. This determined the Qibla, or Muslim direction of prayer. Muslim mathematicians developed spherical trigonometry which was used in these calculations.Around AD 830 Caliph al-Ma'mun commissioned a group of astronomers to test Eratosthenes' calculation of one degree of latitude by using a rope to measure the distance travelled due north or south on flat desert land until they reached a place where the altitude of the North Pole had changed by one degree. The measured value is described in different sources as 66 2/3 miles, 56.5 miles, and 56 miles. The figure Alfraganus used based on these measurements was 56 2/3 miles, giving an Earth circumference of.
In contrast to his predecessors who measured the Earth's circumference by sighting the Sun simultaneously from two different locations, Abu Rayhan al-Biruni developed a new method of using trigonometric calculations based on the angle between a plain and mountain top which yielded simpler measurements of the Earth's circumference and made it possible for it to be measured by a single person from a single location. Al-Biruni's method's motivation was to avoid "walking across hot, dusty deserts" and the idea came to him when he was on top of a tall mountain in India. From the top of the mountain, he sighted the dip angle which, along with the mountain's height, he applied to the law of sines formula. While this was an ingenious new method, Al-Biruni was not aware of atmospheric refraction. To get the true dip angle the measured dip angle needs to be corrected by approximately 1/6, meaning that even with perfect measurement his estimate could only have been accurate to within about 20%.
Muslim astronomers and geographers were aware of magnetic declination by the 15th century, when the Egyptian astronomer 'Abd al-'Aziz al-Wafa'i measured it as 7 degrees from Cairo.
Medieval Europe
Revising the figures attributed to Posidonius, another Greek philosopher determined as the Earth's circumference. This last figure was promulgated by Ptolemy through his world maps. The maps of Ptolemy strongly influenced the cartographers of the Middle Ages. It is probable that Christopher Columbus, using such maps, was led to believe that Asia was only west of Europe.Ptolemy's view was not universal, however, and chapter 20 of Mandeville's Travels supports Eratosthenes' calculation.
It was not until the 16th century that his concept of the Earth's size was revised. During that period the Flemish cartographer, Mercator, made successive reductions in the size of the Mediterranean Sea and all of Europe which had the effect of increasing the size of the earth.
Early modern period
The invention of the telescope and the theodolite and the development of logarithm tables allowed exact triangulation and grade measurement.Europe
In the Carolingian era, scholars discussed Macrobius's view of the antipodes. One of them, the Irish monk Dungal, asserted that the tropical gap between our habitable region and the other habitable region to the south was smaller than Macrobius had believed.In 1505 the cosmographer and explorer Duarte Pacheco Pereira calculated the value of the degree of the meridian arc with a margin of error of only 4%, when the current error at the time varied between 7 and 15%.
Jean Picard performed the first modern meridian arc measurement in 1669–1670. He measured a baseline using wooden rods, a telescope, and logarithms. Jacques Cassini later continued Picard's arc northward to Dunkirk and southward to the Spanish border. Cassini divided the measured arc into two parts, one northward from Paris, another southward. When he computed the length of a degree from both chains, he found that the length of one degree of latitude in the northern part of the chain was shorter than that in the southern part.
This result, if correct, meant that the earth was not a sphere, but a prolate spheroid. However, this contradicted computations by Isaac Newton and Christiaan Huygens. In 1659, Christiaan Huygens was the first to derive the now standard formula for the centrifugal force in his work De vi centrifuga. The formula played a central role in classical mechanics and became known as the second of Newton's laws of motion. Newton's theory of gravitation combined with the rotation of the Earth predicted the Earth to be an oblate spheroid, with a flattening of 1:230.
The issue could be settled by measuring, for a number of points on earth, the relationship between their distance and the angles between their zeniths. On an oblate Earth, the meridional distance corresponding to one degree of latitude will grow toward the poles, as can be demonstrated mathematically.
The French Academy of Sciences dispatched two expeditions. One expedition under Pierre Louis Maupertuis was sent to Torne Valley. The second mission under Pierre Bouguer was sent to what is modern-day Ecuador, near the equator. Their measurements demonstrated an oblate Earth, with a flattening of 1:210. This approximation to the true shape of the Earth became the new reference ellipsoid.
In 1787 the first precise trigonometric survey to be undertaken within Britain was the Anglo-French Survey. Its purpose was to link the Greenwich and Paris' observatories. The survey is very significant as the forerunner of the work of the Ordnance Survey which was founded in 1791, one year after William Roy's death.
Between 1792 and 1798 Pierre Méchain and Jean-Baptiste Delambre surveyed the Paris meridian arc between Dunkirk and Barcelona. They extrapolated from this measurement the distance from the North Pole to the Equator which was 5 130 740 toises. As the metre had to be equal to one ten-million of this distance, it was defined as 0,513074 toises or 443,296 lignes of the Toise of Peru.
Asia and Americas
In South America Bouguer noticed, as did George Everest in the 19th century Great Trigonometric Survey of India, that the astronomical vertical tended to be pulled in the direction of large mountain ranges, due to the gravitational attraction of these huge piles of rock. As this vertical is everywhere perpendicular to the idealized surface of mean sea level, or the geoid, this means that the figure of the Earth is even more irregular than an ellipsoid of revolution. Thus the study of the "undulation of the geoid" became the next great undertaking in the science of studying the figure of the Earth.19th century
In the late 19th century the Mitteleuropäische Gradmessung was established by several central European countries and a Central Bureau was set up at the expense of Prussia, within the Geodetic Institute at Berlin. One of its most important goals was the derivation of an international ellipsoid and a gravity formula which should be optimal not only for Europe but also for the whole world. The Mitteleuropäische Gradmessung was an early predecessor of the International Association of Geodesy one of the constituent sections of the International Union of Geodesy and Geophysics which was founded in 1919.Geodesy and standards of length
In the 18th century geodetic surveys found practical applications in French cartography and in the Anglo-French Survey, which aimed to connect Paris and Greenwich Observatories and led to the Principal Triangulation of Great Britain.Soon after the Anglo-French Survey, the French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to accurately measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona at the longitude of Paris Panthéon. A new unit of length, the metre was introduced – defined as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth's flattening of 1/334.
In 1811 Ferdinand Rudolph Hassler was selected to direct the U. S. coast survey, and sent on a mission to France and England to procure instruments and standards of measurement. The unit of length to which all distances measured in the U. S. coast survey were referred is the French metre, of which Ferdinand Rudolph Hassler had brought a copy in the United States in 1805.
In the early 19th century, the Paris meridian's arc was recalculated with greater precision between Shetland and the Balearic Islands by the French astronomers François Arago and Jean-Baptiste Biot. In 1821 they published their work as a fourth volume following the three volumes of "Bases du système métrique décimal ou mesure de l'arc méridien compris entre les parallèles de Dunkerque et Barcelone" by Delambre and Méchain.
Louis Puissant declared in 1836 in front of the French Academy of Sciences that Delambre and Méchain had made an error in the measurement of the French meridian arc. Some thought that the base of the metric system could be attacked by pointing out some errors that crept into the measurement of the two French scientists. Méchain had even noticed an inaccuracy he did not dare to admit. That is why from 1861 to 1866, Antoine Yvon Villarceau checked the geodesic opérations in eight points of the meridian arc. Some of the errors in the operations of Delambre and Méchain were corrected. In 1866, at the conference of the International Association of Geodesy in Neuchâtel Carlos Ibáñez e Ibáñez de Ibero announced Spain's contribution to the measurement of the French meridian arc. In 1870, François Perrier was in charge of resuming the triangulation between Dunkirk and Barcelona. This new survey of the Paris meridian arc, named West Europe-Africa Meridian-arc by Alexander Ross Clarke, was undertaken in France and in Algeria under the direction of François Perrier from 1870 to his death in 1888. Jean-Antonin-Léon Bassot completed the task in 1896. According to the calculations made at the central bureau of the international association on the great meridian arc extending from the Shetland Islands, through Great Britain, France and Spain to El Aghuat in Algeria, the Earth equatorial radius was 6377935 metres, the ellipticity being assumed as 1/299.15.
In 1860, the Russian Government at the instance of Otto Sturves invited the Governments of Belgium, France, Prussia and England to connect their triangulations in order to measure the length of an arc of parallel in latitude 52° and to test the accuracy of the figure and dimensions of the Earth, as derived from the measurements of arc of meridian. In order to combine the measurements, it was necessary to compare the geodetic standards of length used in the different countries. The British Government invited those of France, Belgium, Prussia, Russia, India, Australia, Austria, Spain, United States and Cape of Good Hope to send their standards to the Ordnance Survey office in Southampton. Notably the standards of France, Spain and United States were based on the metric system, whereas those of Prussia, Belgium and Russia where calibrated against the toise, of which the oldest physical representative was the Toise of Peru. The Toise of Peru had been constructed in 1735 for Bouguer and De La Condamine as their standard of reference in the French Geodesic Mission, conducted in actual Ecuador from 1735 to 1744 in collaboration with the Spanish officers Jorge Juan and Antonio de Ulloa.
Alexander Ross Clarke and Henry James published the first results of the standards' comparisons in 1867. The same year Russia, Spain and Portugal joined the "Europäische Gradmessung" and the General Conference of the association proposed the metre as a uniform length standard for the Arc Measurement and recommended the establishment of an International Bureau of Weights and Measures.
The Europäische Gradmessung decided the creation of an international geodetic standard at the General Conference held in Paris in 1875. The Metre Convention was signed in 1875 in Paris and the International Bureau of Weights and Measures was created under the supervision of the International Committee for Weights and Measures. The first president of the International Committee for Weights and Measures was the Spanish geodesist Carlos Ibáñez e Ibáñez de Ibero. He also was the president of the Permanent Commission of the "Europäische Gradmessung" from 1874 to 1886. In 1886 the association changed name for the International Geodetic Association and Carlos Ibáñez e Ibáñez de Ibero was reelected as president. He remained in this position until his death in 1891. During this period the International Geodetic Association gained worldwide importance with the joining of United States, Mexico, Chile, Argentina and Japan. In 1883 the General Conference of the "Europäische Gradmessung" proposed to select the Greenwich meridian as the prime meridian in the hope that Great Britain would accede to the Metre Convention. Moreover according to the calculations made at the central bureau of the international association on the West Europe-Africa Meridian-arc the meridian of Greenwich was nearer the mean than that of Paris.